Hen's eggs are classified according to their weight under the MAFF (Ministry of Agriculture Forestry and Fisheries) Standards, and generally eggs in M size and L size of the Standards are traded at high prices. In the case of the eggs obtained in the early period of egg-laying, the percentage of the eggs which are less than M size is high. The egg weight increases with aging and the M size and L size eggs begin to be yielded, but the egg producers wish strongly to produce profitable size eggs from the early period of egg-laying for raising profit. Hitherto, the egg weight in the early period of egg-laying has been dependent on the fowl species and the body weight of the hen when it reached adulthood, and in the matter of improvement of feed, it has been generally tried to add methionine or fats/oils, but the effect thereof has been unsatisfactory. It has been reported that the egg weight could be increased by giving feed containing the whole soybean processed by an expander or an extruder (JP-A-10-229829). However, such processed soybean is expensive, and a more efficient technique for increasing the egg weight has been required for practical application.
Further, it is known that the feed intake by fowls reduces with a consequent decrease of egg weight during the period of severe heat in the summer season. This decrease of egg weight leads to a reduction of the percentage of the M size or L size eggs that can be marketed advantageously and a reduction of the gross weight of produced eggs. As for the technique for preventing the decrease of egg weight in the summer season, the addition system of methionine or fats and oils has been employed, but its effect is unsatisfactory.